I don't know how much you all know about the deeper workings of the internet, so here's the long version of the explanation...
The Planet Jailbreak website is hosted by Beyond Unreal. That means the files that make up what you're reading now are stored on their server. The URL for that is http://jailbreak.beyondunreal.com .
The domain name planetjailbreak.com also points to BU's server. You buy domain names like that from companies that add the name to the internet's Domain Name Servers (DNS). It was bought by Daikiki, way back in the mists of time. Now BU have moved their server, the DNS needs to be updated to point to the new server. However, we can't get hold of Daikiki. He's not been around Jailbreak for a very long time.
The upshot of it all is this: it's best if you start using http://jailbreak.beyondunreal.com . The planetjailbreak.com appears to work, but that's because Beyond Unreal have set their old server to forward you, and that's not going to last for ever. We're really sorry about this.
PS. Note also that the JDN's URL is http://mychaeel.no-ip.com/jailbreak/.